16 ANIMAL MECHANICS 



them, and making the weight bear perpendicu- 

 larly upon the walls. When the transverse beam 

 joins the extremities of the rafters, as indicated 

 by the lower outline D, it is called a tie-beam, and 

 is more powerful still in preventing the rafters 

 from pushing out the walls. 



Now when a man bears a burden upon his head, 

 the pressure, or horizontal push, comes upon the 

 lower part of the parietal bones, and if they had 

 not a tie-beam, they would, in fact, be spurred 

 out, and the bones of the head be crushed down. 

 But the temporal bone D, and still more, the 

 sphenoid bone E, by running across the base of 

 the skull, and having their edges lapping over 

 the lower part of the great walls, or the parietal 

 bones, lock in the walls as if they had iron plates, 

 and answer the purpose of the tie-beam in the 

 roof, or the iron plate in the walls. But the con- 

 nection is at the same time so secure, that these 

 bones act equally as a straining-piece, that is, as 

 a piece of timber, preventing the tendency of the 

 sides of the skull to each other. 



It may be said, that the skull is not so much 

 like the wall of a house as like the arch of a 

 bridge : let us then consider it in this light. 



We have here the two parietal bones, separated 

 and resting against each other, so as to form an 

 arch. In the centering, which is the wooden 

 frame for supporting a stone arch while building, 



